Research is being done to determine the mechanism of action of botulinum toxin. The actions of botulinum toxin are being compared to those of other clostridial toxins (e.g., tetanus toxin), and contrasted with those of phospholipase A2 neurotoxins e.g., beta-bungarotoxin). Three mammalian species are being used (mouse, rat, and guinea pig), and two types of preparations from each specie are being studied (phrenic nerve-hemidiaphragm and vagus nerve-sinoatrial node). In addition, work is being done on the electric organ from Electrophorus electricus. The research has two major goals, these being to determine the mechanism by which botulinum toxin causes blockade of transmitter release, and to use the toxin as a research tool to study cholinergic transmission in the autonomic nervous system. Studies aimed at determining mechanisms of action have three major components: 1) the radiolabeling of toxin to do ligand binding experiments, 2) selective modification of toxin with site reactive reagents to determine the identify of active sites, and 3) selective modification of cholinergic membranes to characterize tissue binding sites. Studies dealing with the toxin as a research tool have two major components: 1) the use of labelled toxin as a histological marker to localize post-ganglionic parasympathetic nerve endings, and 2) the use of toxin to produce prolonged blockade of cholinergic transmission, and thus to study the emergence and characteristics of supersensitivity.